This invention relates to cast iron cylinder liners, particularly for use in engines having aluminum cylinder blocks.
It is known in the art relating to cast iron cylinder liners made for use in aluminum block engines to design the liners as cast iron sleeves each with a cylindrical body connected at the top with a radially extending flange. After casting, the outer diameters (ODs) of the liner are precision machined to the proper dimensions. After being press fitted into cylinder bores of an engine, the inner diameters (IDs) of the liners are precision machined. In the assembled engines, the liners are held in place by the retaining force of a cylinder head and associated head gasket engaging upper surfaces of the liners including the flanges. Alternatively, liners may be rough machined on the ODs and cast in place inside aluminum cylinder blocks.
In some applications where the cylinder wall is machined to a thickness of less than 2.5 mm, and particularly where it is 2.0 mm or less, the liner installation and assembly of the engine and the operating conditions in the engine may create undesired stress levels in the liner. The highest stresses generally occur at the location of an arcuate or radiused fillet at the juncture of the outer surface of the cylinder with the lower surface of the radial flange. At this point, the thickness of the cylinder wall is further reduced by an undercut provided at and/or adjacent to the fillet to provide for machining of the outer surface of the cylinder wall with room for tool run-out adjacent to the fillet.
Powertrain development continues to increase engine performance while providing reductions in engine mass and package size. This results in increased engine operational loads acting on the cylinder liners. It is thus desirable to produce thin wall liners modified to either reduce the stress levels and/or increase the strength in the fillet area to provide extended durable operation without significantly modifying the dimensions or the machinability of the liner itself.
The present invention provides a thin wall liner in which the strength of the liner is increased by laser hardening of a portion of the liner wall and the flange adjacent to the fillet at the juncture of the cylindrical wall and the radial flange. The laser energy is preferably applied at or adjacent the location of the fillet by rotating the liner about its axis as the laser energy is applied along the complete perimeter of the fillet portion. Alternatively, the laser beam may be moved around the perimeter of the stationary liner or applied in any other suitable manner.
The laser energy strengthens the portion of the liner wall and flange adjacent to the fillet by modifying the metallurgical structure of the cast iron through phase transformations as known in the art to create a stronger metallurgical structure in the treated portion. The laser treatment may include external quenching of the treated surface after laser heating. However, the treatment may also be conducted without quenching as desired to obtain the best results.
Preferably, the OD of the liner is completely machined before laser hardening. The depth of laser hardening may be varied as necessary to obtain the needed strength at the fillet location. However, it is preferable that the depth of the laser-hardened portion be maintained relatively small to avoid excessive distortion of the machined dimensions of the liner due to the laser treatment.
In a preferred embodiment, the laser hardening pattern extends from the fillet into the flange and into not more than half the thickness of the adjacent liner wall. In an alternative embodiment, the hardened pattern extends into more than half the thickness of the adjacent liner wall to provide increased strength to the fillet portion. In a third embodiment, the laser-hardened portion extends from the fillet through the full thickness of the liner wall adjacent to the fillet. While this provides increased strength, distortion of the upper portion of the liner may require additional machining of the liner bore and fillet area. While this may be done, it is preferred to avoid subsequent machining because machining of hardened portions of the liner causes increased wear of the cutting tools.
Preferably, the fillet is formed as a radius. The adjacent undercut is preferably formed as a tapered or slightly conical area connecting the fillet with a cylindrical portion of the liner wall. However, the fillet may have an arcuate form other than that of a radius and the undercut may be other than conical.
It is known that cylinder bore surfaces have been laser hardened to reduce wear and scuffing of the cylinder surface. However, it is believed that laser treatment of a thin wall liner flange fillet to strengthen the wall for durability has not been previously contemplated or developed.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.